It is known to use holographic non-destructive testing to compare the holographic image of the structure at one strain level with the holographic image from the same viewing angle at another strain level. Exemplary are U.S. Pat. Nos. 3,631,713; 3,644,047; 3,681,970; 3,976,380 and 3,897,995. All these patents employ a beam splitter to provide a reference beam with the exception of the last-mentioned patent in which laser light passes through a photographic plate and is reflected back from the surface being analyzed to provide the reference beam. Devices for reducing spatial frequency in a laser beam employing lenses and an aperture are also known to the art as seen from U.S. Pat. No. 3,532,407. The above-mentioned patents are incorporated herein by reference.
With the exception of U.S. Pat. No. 3,897,995, the apparatus of the above-discussed patents lacks flexibility for readily making holograms of different weld areas and of the same weld area from different angles. While the apparatus of U.S. Pat. No. 3,897,995 provides such flexibility to a considerable degree for an article having a flat surface, it is unsatisfactory for structures such as pipe since the ratio between the reference beam (pure light from the laser) and the object beam (reflected light from the film) varies drastically due to the fact that the roundness of the pipe places different portions of the surface being inspected at widely different distances from the photographic medium. The apparatus of this invention overcomes this problem. Further, this invention eliminates the necessity for adjustment between the laser and the photographic medium each time a hologram is to be made. Much of the vibration problem arising from the separate mounting of the laser mechanism and the structure being tested is eliminated by mounting the apparatus on the structure being tested. By eliminating beam splitting the great bulk of the laser beam can be used for the object being tested and a small amount for the reference beam, for example 10%.